A portion of working machines has a plurality of working members necessary for performing a predetermined operation. The working machine of the type described above is exemplified by a hydraulic excavator. That is, the hydraulic excavator comprises a lower travel body for moving the hydraulic excavator, an upper swing which is placed on the lower travel body such that it can be rotated and a front mechanism comprising a boom, an arm and a bucket. The upper swing has a variety of equipments such as an operation room, a prime mover, a hydraulic pump and the like mounted thereon, the above-described front mechanism being further disposed thereon.
As one of the hydraulic circuit system for use in the working machine of the type described above, there has been a system called "a load sensing system". The system is arranged in such a manner that only the quantity of hydraulic fluid necessary for operating the actuator is delivered from the hydraulic pump by controlling the pump delivery rate such that the level of the pump delivery pressure is higher than the level of the load pressure of the hydraulic actuator.
In recent years, a variety of the load sensing systems have been disclosed. For example, a hydraulic circuit system has been disclosed in JP,A, 57-116965 in which a pressure controller is disposed in the downstream side of a flow control valve for controlling the flow of hydraulic fluid to be supplied to the actuator, the pressure controller being operated in response to the maximum load pressure of a plurality of actuators so as to maintain the differential pressure across the flow control valve at a predetermined value. Another hydraulic circuit system has been disclosed in JP,A, 60-11706 in which it is arranged in such a manner that a pressure compensating valve for maintaining the differential pressure across the flow control valve at a predetermined value is disposed in the upstream side of the flow control valve. In this disclosure, the pressure compensating valve is, as an alternative to the spring, provided with means for causing the pump delivery pressure and the maximum load pressure to interact with each other so as to set the above-described predetermined value by the differential pressure between the levels of the pump delivery pressure and the maximum load pressure. Since the differential pressure across the flow control valve is controlled as described above, the rate of the flow passing through each of the flow control valves, that is, the supply flow rate to each of the actuators can be controlled to the value corresponding to the operation amount (the demanded flow rate) of the operating lever at the time of the combined operation. Furthermore, the speed ratio among a plurality of actuators can be properly controlled so that a smooth combined operation is enabled.
However, the above-described conventional hydraulic circuit system arises the following problems:
In general, the working machine is sometimes used in such a manner that the operation speed of the hydraulic actuator thereof is considerably lowered. For example, in the case of the hydraulic excavator, the following operations are the operations of the type described above: an operation for scraping the surface of the ground, the leveling work, an operation for making a slope (collectively called "fine operations" hereinafter). In the above-described operations, it is apparent that the operation can be easily completed if change in the flow rate to be supplied to the actuator (flow rate passing through the flow control valve) with respect to the operation amount of the operating lever of the actuator is significantly small.
In the fine operation performed by the hydraulic circuit system having no load sensing system, the change in the supply flow rate to the actuator with respect to the operation amount of the operating lever can be reduced by lowering the revolution speed of the prime mover for operating the hydraulic pump and an operator can thereby easily perform the fine operation. However, in the hydraulic circuit system employing the load sensing system, control is performed as described above in such a manner that the control for maintaining the differential pressure across the flow control valve at the predetermined value is performed. Therefore, even if the revolution speed of the prime mover is lowered, the supply flow rate is determined in accordance with the operation amount of the operating lever. Therefore, the change rate of the supply flow rate with respect to the operation amount of the operating lever is not changed, causing the low speed control of the actuator by means of the operating lever to become difficult to be performed. Therefore, the fine operation cannot be conducted easily.
On the other hand, a technology has been disclosed in U.S. Pat. No. 4,487,018 in which an external control signal is supplied to the pressure control means for maintaining the differential pressure across the flow control valve at a predetermined value so as to change the predetermined value. However, the above-described conventional technology has not discussed about the way of making the control signal.
An object of the present invention is to provide a hydraulic circuit system for a working machine capable of performing a fine operation even if the load sensing system is employed therein.